In the legacy Very High Speed Digital Subscriber Line 2 (“VDSL2”) standard, sync symbols occur periodically after every 256 data symbols. On a particular legacy VDSL2 communication line, one or more tones (i.e. subcarriers) transmit a sync symbol including one of two values during a sync symbol period. Typically all tones transmit the same sync symbol value in a given sync symbol period (when defining the value to be the value before any quadrant scrambling). The values may be real or complex. Real values may be represented by “1” or “−1.” The complex values may be may be 00 corresponding to “1+j” or 11 corresponding to “−1−j”. Normally, the value transmitted on all tones is the same from one sync symbol to the next sync symbol. For example, a central office may transmit downstream the value “1” repeatedly on all active tones. When the central office receives an online reconfiguration request (OLR) from a customer-side equipment, the central office may then switch the value to “−1” and transmit “−1” repeatedly to the customer-side equipment. Such a transition is interpreted by the customer-side equipment as an acknowledgment of the OLR. Such an acknowledgment is referred to hereinafter as a sync-flag. A sequence of “1” values and “−1” values that is used to convey sync-flags (or the absence of a sync flag) will be referred to hereinafter as a flag sequence.
In the emerging G.vector amendment to the VDSL2 standard, the current consensus is that pilot signals will be transmitted on sync symbols. For example, pilot sequences may be sent instead of flag sequences downstream on sync symbols during the sync symbol periods to estimate crosstalk on communication lines. For instance, a pilot sequence may be assigned to each communication line of the communication system and the pilot sequences may be sent downstream to the customer-side equipment. At the customer-side equipment, error samples are determined and fed back to the central office. At the central office, the error samples are correlated with the pilot sequences in order to obtain estimates for all of the crosstalk coefficients. The process of obtaining error samples may be repeated as needed to obtain a more accurate crosstalk estimate. These estimates will then be used to cancel crosstalk using precoding. Each symbol of the pilot sequence may be similar in nature to the legacy sync symbol. For instance, each symbol of the pilot sequence may also include the real or complex values of the legacy sync symbols.
A conventional method for transmitting both the pilot sequences and flag sequences on sync symbols consists of time-sharing the sync symbols. For example, odd-numbered sync symbols are used to send arbitrary pilot sequences (e.g., consisting of “1”s and “−1”s). Even-numbered sync symbols are used to communicate flag sequences. That is, the sync-flag is conveyed whenever an even-numbered sync symbol is opposite in sign from the previous even-numbered sync symbol. As a result of time-multiplexing the pilot sequences and flag sequences, sync-flags can only be sent half as often as compared to a system that sends flag sequences on a separate communication line. For example, using time-multiplexing, sync-flags can only be sent after every 513 Discrete Multi-tone (DMT) symbols. Assuming the symbol rate is 4 kHz, the amount of time may be increased by up to 64 ms relative to a system sending only flag sequences.
FIG. 1 illustrates a conventional time division scheme to transmit pilot sequences and flag sequences on the sync symbols. Referring to FIG. 1, the center row of the figure, labeled “sync symbols”, illustrates a complex constellation point sent on eight consecutive sync symbols. The odd symbols are interpreted as pilot values, and are used to transmit the pilot sequence (+1,−1,−1,+1). The even symbols are used to convey four flag values (e.g., labeled “sync bits” in FIG. 1), and are used to transmit the flag sequence (+1, +1, +1, −1). A sync-flag is indicated by the signal change from the 6th sync symbol value to the 8th sync symbol value, or equivalently from the 3rd flag value to the 4th flag value.
The main shortcoming of the time division scheme is evident when the customer-side equipment includes a legacy modem and/or when a communication line is a legacy VDSL2 communication line. A communication line having a legacy modem on the customer-side equipment is hereinafter referred to as a legacy communication line. A communication line having a modem on the customer-side equipment adhering to the G.vector amendment is hereinafter referred to as a G.vector communication line.
When legacy communication lines and G.vector communication lines are both present in a communication system, the sync symbols on the legacy communication line, which conveys only flag values, may interfere with the crosstalk estimation being performed by the G.vector communication lines. For example, if a communication system has four G.vector communication lines, the pilot sequences may be (1,1,1,1), (1,1,−1,−1), (1,−1,1,−1) and (1,−1,−1,1). Each pilot sequence is assigned to each of the four G.vector communication lines, respectively, and transmitted repeatedly on the sync symbols during the sync period. Then to estimate the crosstalk from the second communication line into the third communication line, a sequence of four error samples measured at the customer-side equipment of the third communication line would be correlated with the pilot sequence of (1,1,−1,−1) assigned to the second G.vector communication line. Crosstalk from the other G.vector communication lines would not affect this measurement because of the orthogonality of the pilot sequences.
If the same communication system includes a legacy communication line, the central office would send sync symbols on the legacy communication line by sending flag value “1” repeatedly on all active tones until the central office received an OLR from the customer-side equipment, at which point the central office would acknowledge receipt of the OLR by sending a sync-flag by starting to send the flag value “−1” repeatedly on all active tones. If the resulting flag sequence sent on the legacy communication line happened to be (1,1,−1,−1), coinciding with the pilot sequence sent on the second G.vector communication line, then the crosstalk estimate from the second G.vector communication line into the third G.vector communication line would be corrupted by crosstalk from the legacy communication line.
As a result, the flag sequence on a legacy communication line may coincide with or be strongly correlated with the pilot sequence sent on one of the G.vector communication lines. Therefore, the crosstalk estimate of the G.vector communication lines may be contaminated by the crosstalk from the legacy communication line. For example, if a crosstalk coefficient from the G.vector communication line is g1 and a crosstalk coefficient from the legacy communication line is g2, a mean value of the contaminated crosstalk estimate is g1+ρg2, where ρ is the correlation coefficient between the flag sequence sent on the legacy communication line and the pilot sequence of G.vector communication line, assuming the flag and pilot sequence are transmitted at equal power. Depending on the conditions of the communication system, ρ may be as large as 1. As a result, the error percentage may be as much as 100% error when g1 and g2 have similar magnitudes.